An investigation into the mysterious deaths of a British couple and four other tourists in hotels in northern Thailand found they may have been caused by toxic chemical exposure, it was revealed today.

Pensioners George Everitt, 78, and his wife Eileen, 73, were found dead in their room at the Downtown Inn, Chiang Mai, in February.

Mariam Soraya Vorster, 33, of Berkely, California, was working as a tour guide in the city and died in January after staying at the same hotel.

Mystery: George Everitt and his wife Eileen were found dead in their room in the Downtown Inn, Chiang Mai, Thailand, in February. A government investigation has said exposure to a toxic chemical may have killed them

Victim: Miriam Vorster, 33, was working as a tour guide in the city and died in January after staying at the same hotel as the Everitts

Canadian Bill Mah, from Edmonton, was also found dead in his hotel room of 'suspected natural causes' in January after visiting the Downtown Inn.

And New Zealand backpacker Sarah Carter, 23, died in her fifth floor room at the hotel in February.

A Thai tour guide has also died, and three other tourists fell ill but have since recovered.

The Department of Disease Control (DDC) said on its website today: 'The specific agents that caused the deaths and illnesses in these events cannot be identified, and it cannot be determined exactly how people were exposed to them.'

Results were most revealing in the death of Ms Carter on February 6, her two female companions - both also 23 - who fell ill, but recovered, and a 47-year-old Thai woman who died on February 3.

Illegal chemicals? The Downtown Inn, in the popular tourist destination of Chiang Mai. A probe by a New Zealand TV programme found rooms in the hotel had been sprayed with a potentially lethal toxin

Unexplained: New Zealand backpacker Sarah Carter, 23, left, died in the hotel in February. Right, fears over the use of unsafe chemicals in Thai hotels emerged two years ago after the death of American Jill St Onge in a different resort

The four were 'most likely to have the same cause of illness, probably exposure to some toxic chemical, pesticides or gas,' the report stated.

Mr and Mrs Everitts' death were 'possibly related' to the four 'as they occurred in the same hotel', it adds.

It adds that in the case of Ms Vorster, 'the suspected agent that caused her death is likely to be chemical or biotoxin in nature and it is probable that it might have been a pesticide.'

The final case, a 25-year-old French woman who died on January 19, is not thought to be related to the others. She 'developed fever and other symptoms on January 16 before arriving in Chiang Mai'.

The DDC website added authorities were taking measures to reduce the risks to visitors, but did not give details.

Most had very similar symptoms, including inflammation of the heart, thought to have been caused by food or water contamination.

Thai authorities have continually maintained the deaths linked to the three-star hotel were a coincidence, despite claims of a cover-up by the families of the victims.

A probe by the New Zealand current affairs programme 60 Minutes revealed the hotel rooms has been sprayed with a potentially lethal toxin called pyrophus - which is banned from indoor use in many other countries.

Reporters posing as hotel guests secretly took samples from the room used by Ms Carter and test results found traces of an insecticide chlorpyrifos inside the room.

Fears that some Thai hotels could be using unsafe chemicals emerged two years ago when American Jill St Onge and Norwegian Julie Bergheim died at a different resort.

Their symptoms - severe chest pain, vomiting and fainting - were almost identical to the tourists who died in Chiang Mai.

The popular tourist destination, 430 miles north of Bangkok, is one of Thailand's most culturally significant cities.